Abstract: Based on the software of Ansys Fluent,a three dimensional numerical simulation of ammonia gas jet was carried out to study the characteristics of jet penetration and ambient air entrainment. Both the under-expended area of the ammonia gas jet and the vortex structure were analyzed. The results show that the development of ammonia jet penetration presents a three-staged pattern,which has a strong correlation with jet formed initial vortex and shear layer. The first stage（0＜t＜0.14 ms）is the linear stage where ambient air is hardly entrained at jet boundary and under-expanded area. The second stage（0.14 ms≤t≤0.40 ms）is the transient stage where initial vortex and shear layer have a rapid development while the mass of entrained air increases over 50%（R=8.0）. The third stage（t＞0.40 ms）is the quasi-steady stage where the maximum mass of entrained air decreases by 30%（R=8.0）. Average mass of entrained air increases with the increase of the nozzle pressure ratio R. The average transient mass of entrained air increases when the initial vortex exists at the first and second stage,peaks at which the shear layer fully develops,where mass of entrained air increases to 58%（R=8.0）.